Spectrometers, which find use in measuring the color of samples, operate by dispersing light reflected from or transmitted through a sample into its spectral components and then measuring the intensities of the different spectral components. The spectrometers normally comprise a housing defining an entrance slit to receive light reflected from or transmitted through the sample into the spectrometer housing, a grating positioned within the housing to receive the light passing through the entrance slit, and an exit slit in the housing positioned to transmit a narrow bandwidth of the spectrum to a photodetector which measures the intensity of the component of the spectrum passing through the exit slit. The grating is usually pivotable with respect to the housing to direct different narrow band components of the spectrum through the exit slit for detection.
In some modern spectrometers, instead of employing a pivotable grating and an exit slit, an array of photodetectors is mounted within the spectrometer housing an arranged to detect several components of the spectrum simultaneously. By using an array of photodetectors, the intensity of the components of the spectrum of interest can be obtained in a much shorter time period than is possible with the spectrometer employing a pivotable grating.
In these spectrometers employing a fixed array of photodetectors, the photodetectors are rectangularly shaped and are mounted contiguously so that adjacent photodetectors detect adjacent components in the spectrum. In one device, the photodetectors are about 1 millimeter wide and each detects a component of the spectrum of about 10 nanometers in bandwidth.
Prior to the present invention, the photodetectors in arrays, were all of the same width. However, the dispersion of the spectrum over the array by the grating does not vary linearly with wavelength. As a result, each photodetector detects a bandwidth which is slightly different from the adjacent photodetector, and the difference in the bandwidth detected changes throughout the spectrum from one end of the photodetector array to the other. In the prior art instruments, the bandwidth interval detected by each photodetector was assumed to be the same. This assumption led to errors in color measurements which compare reflectance from, or transmittance through, a sample with standards which have been determined for fixed bandwidth intervals that are carefully calibrated with the color response of the human eye. In order for these comparisons to be precise, the bandwidth intervals used by the color measuring instrument should be the same as the used in the calibrated standards.